The use of polyolefin resins such as polypropylene has become widespread due to their desirable physical properties and their relatively low price. Polyolefinic materials are used extensively in the automobile industry for the manufacture of various external and internal automobile parts, such as bumpers, side mirror supports, interior panels and door handles. It is often desirable to apply a coating to the polyolefin substrate to achieve, for example, a protective and/or decorative coating.
Coating untreated polyolefin substrates is generally difficult because of poor adhesion of coatings to these types of substrates. Polypropylene possesses a low surface tension and low polarity. Hence, it is difficult to ensure that an applied coating will adhere effectively to the surface of a polypropylene substrate.
Two ways to obtain adhesion to polyolefinic substrates are through modifications of the substrate or through the use of adhesion promoting materials. Substrate pretreatment usually involves oxidizing the surface by methods such as flame treatment, gas plasma, corona discharge, UV radiation or chemical means. These processes usually have treatment uniformity problems or may adversely affect the substrate by increasing surface roughness or by overheating. Chlorinated polyolefins have been used as adhesion promoters. They are thought to adhere to the substrate via dispersive forces. The chlorine enhances the solubility in various solvents as well as promoting adhesion of the primer to topcoats by polar interactions.
Environmental concern over the use of organic solvents has become increasingly important to the coating industry. This concern not only extends to preservation of the environment for its own sake, but extends to public safety as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are not only often unpleasant, but also contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to volatile organic compounds (VOCs) which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) established guidelines limiting the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and coating industry which uses organic solvents that are emitted into the atmosphere.
Water-borne chlorinated polyolefin compositions have been developed that are useful as primers for coating polypropylene substrates. These types of compositions usually require the formation of CPO emulsions. These emulsions typically include maleinated CPO, surfactant, amine, water, and in some cases cosolvent. EP 774500 describes a blend of one of these emulsions with an acrylic resin and a crosslinker capable of reacting with the acrylic resin for providing a waterborne coating system for untreated polypropylene. This system is a blend and contains significant amounts of surfactant.
U.S. Pat. No. 5,637,409 describes an aqueous coating composition comprising a blend of surfactant, CPO, monomers, solvent, amine, and water. This dispersion is mixed with initiator and then applied to substrate. The polymerization of the monomers then takes place. This technique is in effect a two component system which contains the undesirable odor and necessary precautions associated with monomer handling. This type of system makes the production of a homogeneous defect-free film difficult.